Generally, discharge tubes wherein gas is enclosed in a tube and a voltage is applied across a pair of electrodes provided at the opposite ends of the tube to cause a discharge between them are used widely in various fields.
FIG. 9 shows a discharge tube 61 which is used with an ignition device C with a series gap of an automobile engine shown in FIG. 10 or the like, and in the discharge tube 61, an electrode 63 in the form of a needle and another electrode 64 in the form of a flat plate are provided at the opposite ends of a cylindrical casing 62. Inert gas is enclosed in the discharge tube 61. The discharge tube 61 acts as a series gap S of the ignition device C, and as the discharge voltage at the series gap S is maintained high to some degree and the voltage across the series gap after discharging is applied to an electrode of an ignition plug P, an ignition voltage necessary for the ignition plug P is obtained without having a significant influence of an electric shunt circuit which may be caused by carbon sticking to the ignition plug P or the like.
However, since the electrode 63 of the discharge tube 61 on the side to which a voltage of an ignition coil 65 is applied is formed into a needle-like configuration as shown in FIGS. 10 and 11 and as described hereinabove, there are problems that a non-uniform electric field is formed between the electrodes 63 and 64 of the discharge tube 61 and readily causes discharging and that the discharge voltage V.sub.1 of a voltage characteristic illustrated in FIG. 12 does not present a very high level. Accordingly, in order to raise the discharge voltage V.sub.1 of the discharge tube 61 to some degree, the distance between the needle-formed electrode 63 and the flat plate-formed electrode 64, that is, the series gap S, is increased to some degree. However, where the series gap S is increased in this manner, there is a problem that the configuration of the entire discharge tube is increased, and there is another problem that a discharge maintaining voltage V.sub.2 of FIG. 12 becomes high and the energy loss during discharging is increased. Also, there is a problem that, since the electrode surface area of the needle-shaped electrode 63 is small, the influence of exhaustion of the electrode by discharging is great and the durability is not so long.
Thus, it may seem recommendable to form the electrodes of the discharge tube 61 as a pair of substantially parallel plate electrodes (Rogosky electrodes) which are often used in experiments of a discharge phenomenon and wherein an end face has a flat face configuration and a circumferential edge portion around the end face is rounded into a curved face configuration in order to approximate an electric field between the electrodes of the discharge tube to a uniform electric field so as to cause a discharge less readily to allow the discharge voltage V.sub.1 shown in FIG. 12 to be raised and also to reduce the distance between the electrodes to lower the discharge maintaining voltage V.sub.2 to decrease the energy loss.
However, such Rogosky electrode has a characteristic that it is readily influenced by a quantity of electrons floating between the electrodes and electrons are not discharged from the electrodes, and there is a problem that the discharge voltage V.sub.1 is not stabilized and is influenced by a frequency of discharges.